1. Field of the Invention
The present invention relates to a spark plug used for ignition of an internal combustion engine.
2. Description of the Related Art
As for spark plugs used for ignition of an internal combustion engine such as an automobile engine, many spark plugs of the type having a center electrode formed to protrude from a leading end of a metal shell have been proposed, as compared with a conventional spark plug. This is for the following reason. Generally, when this type spark plug is attached to an internal combustion engine such as an automobile engine, a spark discharge gap formed between the center electrode and a ground electrode is provided inside a combustion chamber. Consequently, the ignitability of the spark plug can be improved (see Japanese Patent Laid-Open No. 153677/1981).
A large number of proposals have been also made for spark plugs of the type including a ground electrode having one end joined to a metal shell, and a precious metal tip bonded to the vicinity of the other end (the other end portion) of the ground electrode opposite the one end of the ground electrode. This is for the following reason. As described above, this type of spark plug is formed so that the spark discharge gap protrudes into the combustion chamber to improve ignitability of the spark plug. Accordingly, the ground electrode for forming the spark discharge gap is exposed to a high temperature (see Japanese Patent Laid-Open No. 2001-345162).
In the aforementioned spark plug, there is however a possibility that a spark generated in the spark discharge gap may be blown under the influence of a swirling flow or the like because the spark discharge gap is provided inside the combustion chamber. As a result, flying sparks may attack a joint portion between the ground electrode body and the precious metal tip because the spark shifts from a leading end surface of the precious metal tip. As a result, there is a possibility that the precious metal tip may drop out of or rather separate from the ground electrode body because the joint portion is worn out.
It is therefore effective to increase the axial-direction shortest distance between the joint portion and the leading end surface (opposite the center electrode) of the precious metal tip bonded to the ground electrode. According to this configuration, the spark can be caught in side surfaces of the tip even in the case where the spark is blown by the swirling flow. Accordingly, flying sparks can be prevented from attacking the joint portion, to prevent separation of the precious metal tip from the ground electrode body.
In a spark plug formed so that the axial-direction shortest distance between the joint portion and the leading end surface of the precious metal tip is increased as described above, the distance (hereinafter referred to as a protrusion height) between the leading end surface of the precious metal tip and the inner circumferential surface of the ground electrode body becomes large. In such spark plug, however, there is a possibility that the wear resistance of the precious metal tip will be lowered. This is because the heat capacity of the precious metal tip becomes so large that the precious metal tip assumes a high temperature.